Conventionally, various circuits for protecting against an Electrostatic Discharge (ESD) have been proposed. As used herein, ESD means a discharge of electricity from a person or a machine charged with electricity to a semiconductor device or a discharge of electricity from a semiconductor device charged with electricity to a ground potential or the like.
When an ESD is received by a semiconductor device, an electric charge flows into the semiconductor device from terminals of the semiconductor device, and this electric charge generates a high voltage in the semiconductor device which may cause an insulation breakdown in an internal element of the semiconductor device and/or failure of the semiconductor device.
A typical example of an electrostatic protection circuit is an RCT (RC Triggered) MOS circuit. The RCT electrostatic protection circuit includes a trigger circuit, which includes a resistor and a capacitor connected in series between power source terminals, and a MOS transistor is driven using a voltage from a connection point (node) between the resistor and the capacitor as a trigger signal. The MOS transistor is switched to an ON state for discharging the electrostatic charge according to a time constant of the trigger circuit. That is, once the voltage at the connection node exceeds a threshold voltage of the MOS transistor, the MOS transistor is switched to a conductive state. The trigger signal maintains the MOS transistor in a conductive state while the capacitor remains charged above the threshold level of the MOS transistor. Hence, it is necessary to set the time constant of the trigger circuit such that an ESD surge can be sufficiently discharged. However, when the time constant becomes large, the trigger circuit will responds to fluctuations in the power supply voltage during normal operation of the internal circuit and hence, there is a possibility that the protective MOS transistor is erroneously operated when an ESD surge is not generated. When the MOS transistor is erroneously operated the power source voltage does not sufficiently rise to an intended value thus causing an operation failure in the internal circuit. Further, when the time constant is set such that that the protective MOS transistor is held in an ON state for a long time, there exists a possibility that the MOS transistor will break down.